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Setting an industrial heat-treating oven to 400 degrees without checking the unit could ruin an entire production run — 400∘C400^\circ\text{C} is a scorching 752∘F752^\circ\text{F}, hot enough to melt zinc and warp precision steel.
In technical trades, matching technical drawings from international clients to American shop tools requires converting between Fahrenheit (∘F^\circ\text{F}) and Celsius (∘C^\circ\text{C}) quickly and accurately.

The Two Scales

Daniel Gabriel Fahrenheit built the first precise mercury thermometer in 1724, while Anders Celsius introduced the 100-step scale between water's freezing and boiling points in 1742.
Water freezes at 0∘C0^\circ\text{C} (32∘F32^\circ\text{F}) and boils at 100∘C100^\circ\text{C} (212∘F212^\circ\text{F}). That means a span of 100∘C100^\circ\text{C} equals a span of 180∘F180^\circ\text{F}, giving a conversion ratio of 180100=95=1.8\frac{180}{100} = \frac{9}{5} = 1.8.
How do we turn this fixed ratio into a dependable conversion formula on the shop floor?

Converting Celsius to Fahrenheit

Because each Celsius degree is 1.81.8 times larger than a Fahrenheit degree, we first multiply the Celsius value by 1.81.8 (or 95\frac{9}{5}), then add 3232 to account for Fahrenheit's higher zero point: F=1.8C+32F = 1.8C + 32
Suppose a specification sheet specifies preheating a pipe joint to 220∘C220^\circ\text{C} before welding, but your digital pyrometer reads in ∘F^\circ\text{F}.
What if you need to run the process in reverse and program a metric machine using standard American blueprints?

Converting Fahrenheit to Celsius